With rapid development of electronics, communications and computer industries, portable electronic communication equipments such as camcorders, mobile phones or notebook computers develop remarkably. Accordingly, the demand for a lithium secondary battery as a power source for driving the portable electronic communication equipments is increasing day by day. In particular, in application of electric vehicles, uninterruptible power supplies, motor tools or artificial satellites, research and development of the lithium secondary battery as an environmentally friendly power source is lively made inside and outside of the country including Japan, Europe and U.S.A.
Currently, an anode active material for a lithium secondary battery includes crystalline carbon such as natural graphite or artificial graphite, and amorphous carbon such as non-graphitizable carbon or graphitizable carbon.
The natural graphite has advantages of low price, a flat discharge curve at a negative potential and excellent initial discharge capacity. However, charge/discharge efficiency and charge/discharge capacity reduce remarkably while charge and discharge cycles are repeated.
Mesophase graphite has a spherical particle shape and allows high density filling, and thus is capable of improving an energy density per volume of a battery and exhibits excellence in forming an electrode plate. However, the mesophase graphite has a disadvantage of a low reversible capacity.
The non-graphitizable carbon has advantages of excellent safety and high capacity. However, the non-graphitizable carbon has smaller size than graphitizable carbon, and has a micropore, consequently low density, thus causing irregular particle shape and particle size after a pulverizing process. By these reasons, the non-graphitizable carbon is difficult to be widely applied to a battery.
And, to meet the demand for safety and high capacity, a recent attention is given to a lithium titanium oxide. The lithium titanium oxide has a spinel-type stable structure, and thus is evaluated as one of materials capable of improving safety. The use of the lithium titanium oxide as an anode active material allows flatness of a potential curve, excellent charge and discharge cycle life, improved high rate characteristics and power characteristics, and excellent durability. However, in the case that lithium titanium oxide is used singularly, battery characteristics are reduced due to a low average voltage.
Therefore, various methods have been suggested to solve the problems of the conventional anode active material. So far, however, there is no report of such an anode active material evaluated as it improves electrical characteristics and safety of a lithium secondary battery.
For example, Korean Patent Registration No. 10-0666822 discloses a method for surface-coating carbon with a metal or metalloid for high capacity and high efficiency of a battery.
Korean Patent Registration No. 10-0433822 discloses a method for surface-coating a carbon active material with a metal or metal oxide to improve conductivity, high rate charge and discharge characteristics and cycle life.
Korean Laid-open Patent Publication No. 10-2007-0078536 discloses a method for coating natural graphite with a low crystallinity carbon material.
Korean Laid-open Patent Publication No. 10-2006-0106761 discloses a method for adding graphite or carbon black to lithium titanium oxide so as to prevent overcharge.
However, the methods suggested in the above-mentioned prior arts are evaluated as not sufficiently exhibiting effects of maintaining electrical characteristics well and improving safety of a lithium secondary battery.
Therefore, it requires to suggest an anode active material capable of maintaining excellent battery characteristics and exhibiting excellent safety and a method for preparing the anode active material with excellent reproducibility and productivity.